The following terminology is exemplary, and not intended to be limiting in any way. The text “capacitive sensing element”, and variants thereof, generally refers to one or more data elements of any kind, including information sensed with respect to individual locations. For example and without limitation, a capacitive sensing element can include data or other information with respect to a relatively small region of a fingerprint image. After reading this application, those skilled in the art would recognize that these statements of terminology would be applicable to techniques, methods, physical elements, and systems (whether currently known or otherwise), including extensions thereof inferred or inferable by those skilled in the art after reading this application.
Fingerprint sensing technology has become widespread in use and is often used to provide secure access to sensitive electronic devices and/or data. Generally, capacitive fingerprint sensors may be used to determine an image of a fingerprint through measuring capacitance through each capacitive sensing element of a capacitive sensor. The higher the capacitance, the nearer the surface of an adjacent or overlying finger to the capacitive sensing element. Thus, fingerprint ridges provide a higher capacitance in an underlying capacitive sensing element than do fingerprint valleys.
Capacitive fingerprint sensors come in at least two varieties, namely active and passive. Active capacitive sensors are often used in electronic devices to provide biometric security and identification of users.
Active capacitive sensors initially excite the epidermis of the sensed finger. Capacitance to the epidermis is measured at each capacitive sensing element. As one example, capacitance may be measured or determined by measuring a capacitive sensing element's voltage and/or charge during a low voltage phase and a high voltage phase of a modulation frequency for the capacitive sensing element array. The difference in voltages may be used to determine capacitance. One example of an active capacitive sensor is shown in FIG. 1.
As shown in FIG. 1, the active fingerprint sensor may include both capacitive sensing element array 102 on sensor chip 100 and drive ring 104. The voltage of capacitive sensing element array 102 is not directly driven or modulated, but instead drive ring 104 is modulated by drive amplifier 106. This, in turn, excites finger 108 and the voltage and/or charge at each capacitive sensing element of capacitive sensing element array 102 varies as drive ring 104 is modulated since finger's 108 voltage potential changes with the modulation of drive ring 104.
In such a sensor, the voltage that may be applied to the drive ring may be limited. Commonly, the drive ring voltage is no more than 4 volts peak-to-peak. Voltages above this may excite the finger to too high a voltage; this excessive excitation may be detected by a person as a “tingling” or uncomfortable feeling in their finger. Although the exact voltage at which one can sense the tingling varies from person to person, a 4 volt peak-to-peak voltage is generally considered as the threshold beyond which the feeling is noticeable.
Since the drive ring's voltage is restricted to avoid user perception, the thickness of any dielectric overlaying the sensor may also be limited. The thicker the dielectric between sensor pad and finger, the more attenuated the resulting capacitance and the blurrier the fingerprint image becomes. For dielectrics having a thickness or more than approximately 100 microns, the fingerprint image may become unreliable.
Another limitation arises when other parts of the user's finger or hand or body may capacitively couple through earth ground to the system, or directly to the system ground when touching other parts of the system. This capacitive coupling from the user to the system may be highly variable depending on how the user is touching the device. This parasitic coupling attenuates the voltage that the drive ring is able drive into the user's finger, and as such reduces the signal. The attenuation may be highly variable depending on how the user is touching the device.